Why would someone here take the time to explain it when all you have to do is search google? It seems to me that the real reason for your question is to increase post count.
This thread has already been posted numerous times here in DP. Just hit the () button and report it to the mods. DP noobs should really learn to read first then post second. OR Google first then ask second.
In short cloacking manipulating pages and show different output results to real visitors and search engines
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